ptp: Split out PTP_SYS_OFFSET_EXTENDED ioctl code

Continue the ptp_ioctl() cleanup by splitting out the
PTP_SYS_OFFSET_EXTENDED ioctl code into a helper function.

Convert it to __free() to avoid gotos.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250625115133.050445505@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>